The Borg's biggest advantage is their ability to adapt to an enemy's weapons. Besides the fact they probably have tens of thousands of cubes.
They dont even waste energy on non threats. They ignore them. They only take action when it's proven
that a person or ship can do damage to them.
The Dominion cant afford to react that way--one cube and they'll send a huge fleet which may or may not be wiped out.
Second biggest advantage is assimilation-- if they get the right Vorta scientists--it's over.
One of the Founders biggest advantages is that they seemed to be experts at genetics--I can see them devising some type of virus to infect the entire Borg.
But if the Borg can adapt to that ............?